Lowercase the "apply WebVTT cue settings" definition

For consistency with e.g. "collect WebVTT cue timings and settings"
Refactor how and when "apply WebVTT cue settings" is invoked

This factors out the common "Let nodes be" and most importantly makes
the algorithm actually return the CSS boxes.
Drop 'cue' from 'A cue line position/alignment'

No other terms in this section except 'A cue box' include 'cue'.
Quote strings in the syntax section for consistency

Compare with e.g. the "align" setting.
Merge pull request #93 from foolip/quote-strings

Quote strings in the syntax section for consistency
Merge pull request #91 from foolip/a-cue-line

Drop 'cue' from 'A cue line position/alignment'
Merge pull request #82 from foolip/reorganize-rendering

Refactor how and when "apply WebVTT cue settings" is invoked
Changes to the handling of "position" and "positionAlign".

Introduce both an "auto" value for "position" and "positionAlign"
settings which depends on the "text alignment".

Introduce a "computed text position" and "computed text position
alignment" algorithm such that these can be set by script through VTTCue.

This results in some simplifications in the rendering algorithm, in
particular we now don't need positionSet and positionAlignSet variables
any longer.
Apply position and positionAlign fixes also to regions.